IC recommendation....

[digi2t]

Question for the "way smarter than me" folks here...

I'm working on my Ampeg G-212. This time I'm upgrading the pre-amp side of things, new caps and such. R.G. and PRR's help on the power amp side of things was amazing. That end is rock solid now.

As for the preamp IC's, what would be a suitable replacement for the uA709C? I can get some LM709CN's, but would the MC1709 or NE5534 be better?

As for the RC4739, I'm going to go with a TL072. I figured out that I can plug the 8 pin "072", pin 1 to pin 1, into the 14 pin socket, and use 5 jumpers to the NC pins to feed the TL072. The 3 remaining pins line up with the RC4739. Maybe someone has a suggestion here as well?

Opinions???

Cheers,
Dino

[PRR]

> replacement for the uA709C

Where's the schematic?

The '709 is, by modern standards, unique and quirky. There is no "always works" replacement. I must point out that in some ways, the '709 can be _better_ than many later opamps.

The '739 has slipped my mind. I see it is a '741 topology in an odd pinning and claims low noise.

[digi2t]

> replacement for the uA709C

Where's the schematic?

The '709 is, by modern standards, unique and quirky. There is no "always works" replacement. I must point out that in some ways, the '709 can be _better_ than many later opamps.

The '739 has slipped my mind. I see it is a '741 topology in an odd pinning and claims low noise.

WHOOPS!!! Sorry Paul. I should have known better.  Thanks for the bitch slap, I'm feeling MUCH better now.

Here's the diagram of the preamp side, it's IC1 (RC4739), and IC2 (uA709C), at the top of the schematic.



I did find an upgrade/retrofit memo for the Auditronics 501, replacing the uA709 for the "improved performance" NE5534. It also includes a diagram, showing the removal of the compensating caps and resistor. Would the NE5534 be worthwhile here?

I have tried the TL072 in place of the RC4739. It's an easy mod, simply using 5 jumpers to the NC pins of the socket to transfer the lower pins to the 072. Soundwise, switching between the two channels, it's seems a bit brighter, punchier now.

[PRR]

There's nothing super-sexy happening here. Like the early VT40 and SVT, it is Ampeg-unique tone controls around very ordinary amplifiers.

Not sure there is much "improvement" on tap.

IC1a may as well be a TL072-type. The hiss may be lower. (There may be lower-hiss JFET input amps available now.) You can also then raise R1 to 1Meg or 5Meg, which may bring-out the guitar overtones a bit more.

IC1b could be about anything. '4739 is fine here, though if you replace IC1a then you'd logically use a dual.

IC2 is not doing anything special; it must have been the year that '709 was going out of style thus was the cheapest thing around. Remove R21 C16 C17, then whatever fits (you probably need an adapter or a hack). NE5534 would be very fine, but note that '5534 needs a small compensation cap for low gains. TL071 would be fine too.

IC103 reverb return begs for something better than '709. '5534 would be noticably lower hiss.

The tremolo is fascinatingly primitive. Don't touch it, it might quit working.

[digi2t]

Thanks Paul.

All info duly noted. I`ll try your suggestions, and report my findings.

Cheers,
Dino

[digi2t]

OK Paul, while I'm waiting on the 5534's, I did try the R1 resistor mod you suggested. So far, I'm only working with channel 1, so I can compare to channel 2 each time I mod something. I put a 2.2M pot in series with the resistor, so I could listen to what was going on. At the far end of the dial, 2.2M + 100K, it's actually very nice. Much of the hum/hiss that's left, with the 072 in the IC1 spot, gets really cut down. This also further increases the punchiness of the sound, compared to the original arrangement of channel 2. I think a 2.2M resistor will get a permanent spot here. So far, just with the TL072, and 2.2M resistor mod, A/B'ing the two channels is like night and day. At identical settings, channel 1 is cleaner and punchier, while channel 2 is it's much duller twin. With my WEM Project V fuzz pedal plugged into channel 1, it's friggin' amazing.

There is one thing that bugs me though, and this applies to both channels. With just a straight guitar plugged in (no effects in between), the mid-range control is giving me distortion as I crank it up. The way the pot works is like this; the dial is labeled in an EQ type format. -20dB at one end, 0 in the middle, and +20dB at the other end. The three position switch overhead decides which frequency range the pot will EQ, 300hZ, 800hZ, or 3000hZ. Consequently, having the pot at 0, and switching between the three ranges, yeild no tonal difference. The problem is this, as long as I keep the mid control below -12dB, in any of the three ranges, the sound is great. Even at max volume (VERY LOUD!!!), a straight guitar plugged in, it's really clean. But, if I dial in more mids, the amp starts to distort. Not a really dirty type of distortion, more of an overdrive type sound, but then some notes in the higher registers also start to exhibit only what I can describe as a nasty undertone. Not really a ghost note, more of a hum on the initial pick attack, that fades quickly, before the note decays.

I thought at first that it may have been the speakers, so I swapped out the original Eminence ones, for a set of Texas Heat's (2 x 12's @ 8 ohms total, 150 watts each), but I'm still getting the same result.

I've posted the complete schematic below.

Is the mid tone control pushing the preamp, or power amp section too hard? Is there a way to "tone down" the mid control so I can use more of the dial? Electro's on the preamp board drying out maybe?




Cheers,
Dino

[PRR]

> Electro's on the preamp board drying out maybe?

That's a thought.

BTW: all electro caps in the signal path (not power pins) in this amp should be NON-polar. Polarized will work for a while, and are cheaper, which is why they used polar. Who wants an amp to live forever? Not the maker who hopes to sell you another one someday. If you are not falling for this game, and maintaining the amp past the first decade, your labor is worth more than the few cents saved. Stuff non-polar.

[R O Tiree]

They haven't exactly gone overboard with the headroom on those electros, either... It really annoys me when I see them "gooped" to make them more difficult to replace.

The tone control is a Baxandall topology (there are many variations on the theme). They could have just gone with putting the Mid control in there as well, but the disadvantage is that the Mid and Treble controls interact with each other in a predictable but fiddly to use way. They made a virtue out of a glitch by breaking the Mid control out into that centre-frequency-selectable one in the previous stage, thus removing the potential problem completely. Neat solution

[digi2t]

Alrighty then, non-polars it is. You're absolutely right Paul, I'm most definitely trying to keep this puppy in shape past it's first decade... or four.  I love this amp to death, it's super clean (sound), and super strong (loud). Really, I've never played through anything else like it. I cherish it, as much as my Gibson G-100. Nice clean, no-frills amps. I think I'm more attached to the Gibson though, only because Jose Feliciano used it back in the early 80's as a rental, at Place des Arts, here in Montreal.

So from what I see, on the preamp board, C2, C3, C7, and C9 will be swapped for non-polars, is that correct? I'm just not sure now if I should do the same for the Reverb/Tremolo board, and which ones I would change, if any.

Cheers,
Dino

[digi2t]

Hmmm...

I believe I have a problem. If I wish to reduce the gain of the preamp, would I be increasing R3 (680R)?

I've got the TL072 and NE5534 in there now, I removed the compensating caps and resistor (C16, C17, and R21). From what info I've found, I need a cap across pins 5 and 8 on the 5534, between 12pF and 22pF? Is that correct?

Problem is that I've got too much gain on tap now from the preamp section. If I turn up the volume more than half, the amp oscillates. Actually, I slightly toasted the R224 (10R) resistor. Bit of smoke, but no damage, thank God. I was quick on the power switch, but the 10R now measures 15.1R  . Replaced the resistor, everything is fine at that end.

[PRR]

The layout was intended for the old/slow amps. You got a couple octaves more supersonic response, enough to oscillate.

> cap across pins 5 and 8 on the 5534, between 12pF and 22pF?

Go 27pFd. Keep leads very short.

Put 150pFd-220pFd across R2 so that the preamp stage gain falls to unity above the audio band.

Change C202 to 500pFd-1,000pFd for futher supersonic cut.

[digi2t]

Thanks Paul, will do. I`ll let you know how it goes.

Thanks for keeping me on the learning curve  .

[digi2t]

OK, the latest news.

I tried the caps you recommended, but to no avail. Turning the volume higher than, say 7, and the oscillation is still there.

But...

I left the 220pF cap in, and tried an OPA2134 in place of the TL072. Well lo and behold, it works! Crank the volume right up, and no oscillation. Really clean, and less hiss than ever before. I'm not really sure what's going on here, but I guess that's going to stay in there, along with the NE5534. I didn't have a 27pF cap, so I have a 33pF in there right now and it seems to do the job.

I found that the nasty undertone on the pick attack was actually coming from the power amp section. I sort of had my doubts that it was coming from the preamp on this one. When I had repaired the power amp section, I had also replaced the Q201/2/3/6/7 transistors as well. I figured, "An ounce of prevention", at the time. I thought that 2N5551/5401's would be fine, but it seems that they don't work well here. Research lead me to try BD138/9's here instead. Another problem solved! Undertone is completely gone.

I will most definitely increase the C202 as well. The modded preamp is very bright now, compared to the original configuration. I don't even have the Bright switch on, and it's slightly brighter than the original. I think 500pF will be plenty here. That, and a new 10R resistor, to replace the two parallel 22R's that I'm cheating with. 

[PRR]

> new 10R resistor, to replace the two parallel 22R's that I'm cheating with.

Parallel is fine here, BUT they must not be wire-wound!!

Also they must handle a lot of power in unusual conditions. Normally they dissipate nothing, but if the amp (or preamp) oscillates supersonic, they must survive all the power of the amp (OTOH amp power drops above the audio band because the fat transistors won't go that fast).

However most big resistors are wirewound.

Traditionally we used 2W Carbon Composition. These days Carbon Film is much more available, though you may have to parallel a few to get 2W, and you should be generous because Film will not stand short over-loads as well as Composition. Or you can look for "non-inductive" resistors. They exist up to quite large values (thousands of Watts!) but are fairly hard to find.

10 ohms, 11 ohms, 8 ohms... not too critical. Just _some_ load so the amp load does not rise at higher frequencies (such as speaker impedance rise). Indeed a smaller "dummy speaker" resistor rated non-inductive may be good. Keep leads short, a few inches max.

[digi2t]

Thanks for the heads up on the resistor. I managed to find a 10R - 2W carbon comp. It will be in there as soon as I get it.

What's your take on why the OPA2134 works, and the TL072 doesn't? Right now, I have a 220pF across R2, and a 33pF as the compensation for the 5534. I've gone back to the 100K at R1. I find if weird that the OPA works fine, yet the TL craps out when you get past 7 on the volume.

[digi2t]

Well, the "Dream Team" right now seems to be the combination of an OPA2134 in the IC1 slot, and an OPA134 in IC2. I've got a 220pF silver mica across R1, and the compensation caps and resistor have been removed from around IC2. On the power side, I tried different caps on the input side, but have opted to stay with the 150pF. It's really not as harsh as I thought it would be. Oh yeah, all the electros in the audio path are now non-polars.

I did a "blind taste test", running the updated channel 1 versus the original channel 2, by my son. He remarked that he really favored channel 1. It's brighter, but not overly so. It's cleaner too, yet retains a certain organic feel. Using the neck single coil on my Strat is really cool through it. Just the right balance of punch, pop, and boom. Throw my WEM Project V fuzz into the mix, and this sucker goes nuclear! I mean, tie your tuque down with some bailing wire man, and are those your shoes over there?

As for IC103, an OPA134 will most likely find a home here as well. Haven't tried it yet, but if the results on the preamp side is any indication, then Bob should be my uncle.

A great big fat THANKS to you Paul, for your insights and suggestions on this. I'm really a fish out of water where amps are concerned, but with your help, this puppy is really on a higher level now. I've fallen in love with this amp, all over again.